1. Field of the Invention
The present invention relates to the field of computerized analysis of digitized medical images such as chest X-ray images, for example, which is used for the enhancement of subtle interval changes occurring between a pair of images corresponding to a given patient, one of which has been produced prior to the other so that changes between the two images can be detected using digital image processing techniques.
2. Discussion of the Related Art
In clinical situations, comparison with a previous chest radiograph of the same patient has been considered important. Most of postero-anterior (P-A) chest radiographs are observed side-by-side with previous radiographs. Comparison readings help radiologists to identify abnormalities and to determine their significance. They are also important in detecting temporal changes in known lesions so that the effect of treatment can be evaluated. However, important interval changes can be missed by radiologists even when they observe serial chest radiographs, as has been described in Greene, "Missed Lung Nodules . . . ", Radiology, Vol. 182, pp. 1, 8-9, 1992, and Austin, "Missed Bronchogenic Carcinoma . . . ", Radiology, Vol. 182, p. 1, 115-122. The reason for such errors is partly due to the difficulty in comparing two radiographs by scanning back and forth between them, and also due to differences in density, contrast or patient positioning between the two radiographs. Interval changes of a patient having a number of abnormalities often can be overlooked because some abnormalities are camouflaged by other abnormalities not showing any change. Therefore, it is important to obtain information related to interval changes in temporally sequential chest radiographs to alert radiologists and improve the possibilities of detecting important changes in pathology. An interval change is defined here as a pathological change which has occurred after the previous examination and before the current examination.
In general, chest radiographs are hardly reproducible in terms of patient positioning, x-ray projection, and other exposure conditions. Also, respiration and cardiac pulsation of a patient will usually be at different phases for the two images, which results in changes in the size and the shape of the lungs, diaphragm and heart. Therefore, an image registration technique is needed so that a pixel on one image can be subtracted from that corresponding to the identical anatomic structure on the other image. In addition, the fact that radiographs correspond to two-dimensional projections of three-dimensional objects prevents the accurate registration of images in a simple manner. FIG. 12 illustrates various sources of misregistration in pairs of temporally sequential P-A chest images, which have been determined by studying more than 40 pairs of images. Most pairs have misregistration due to a combination of these sources.